In the electronics industry multilayer circuit boards are prepared by bonding a layer of incompletely cured thermosetting resin reinforced with fiberglass between layers of a fully cured print and etched laminate. For a four-layer epoxy based circuit board, first an epoxy coated fiberglass composite is laminated with thin copper foil on both sides. On one side of the laminate, the copper is patterned using conventional printed circuit board manufacturing processes. The side containing the patterned copper layer is referred to as the inner layer. Two laminates having the inner layers facing each other are then bonded together typically using an FR-4 prepreg (a flame retarded partially cured sheet of epoxy coated fiberglass that has no copper foil cladding). The inner layers are then bonded together using the partially cured epoxy as an adhesive layer by pressing the construction together in a press at temperatures such as 360° F. (182° C.) for two hours at 200 psi, thereby fully curing the epoxy FR-4 adhesive layer. A composite is thereby created having non-pattered copper layers at the surfaces and patterned inner layers being separated by the adhesive layer. The top and bottom non-patterned copper layers (the outer layers) can then be print and etched yielding a four-layer circuit board.
One drawback of using many conventional thermosetting resins as the adhesive layer is the poor electrical properties of the bonding adhesive layer. Epoxy based thermosetting resin, for example, has poor electrical loss characteristics in the 1-100 gigahertz range. For very long trace lengths, signal degradation forces the use of lower loss dielectrics. This is increasingly becoming the case for high speed digital applications (routers, backplanes, motherboards and daughter boards). For the RF and mm wave frequencies, polytetrafluoroethylene (PTFE) based materials are traditionally used to prevent signal loss. PTFE based materials have been available for a long time for the most demanding low signal loss applications but have been avoided for cost considerations. Conventional thermosetting resins have too high a loss tangent at the high frequencies and are nearing their ultimate limits at 2.5 GHz. As frequencies extend to the 5 and 10 GHz range, it is likely that epoxy resins will be replaced by higher performing materials. Suppliers of epoxy laminate have been reducing the loss tangent of their products by switching to lower loss polyphenylene oxide based polymers and ceramic fillers. Typical PTFE products have 0.002-0.004 loss tangents versus 0.007-0.014 for epoxies and related materials (10 GHz). As signal integrity drives the use of higher performing materials, epoxy based solutions will eventually fall short even with high loadings of ceramics.
An alternative solution is the use of expanded PTFE that has been filled with epoxy and ceramic, thereby diluting the concentration of the higher loss epoxy component. This combination of epoxy, ceramic, and PTFE results in a sufficiently low loss product to be acceptable for high speed digital applications. The downside is that the expanded PTFE based solution is quite expensive and there are issues of dimensional movement that becomes significant with increasing layer count. U.S. Pat. Nos. 4,985,296; 4,996,097; 5,538,756; and 5,512,360 awarded to W. L. Gore describe the use of a thermosetting resin impregnated into an expanded PTFE web. These patents teach the use of incorporating ceramic in the PTFE expanded web manufacture and/or part of the non-fluorinated adhesive resin system to obtain low loss materials.
Ceramic filled resin systems based on polybutadiene-woven fiberglass based prepregs, both filled and unfilled with flame retardant additives, are known to be relatively low loss materials (U.S. Pat. No. 5,571,609). These materials suffer from the inconsistent quality of the peroxy cured rubber system and the poor bond strengths of the cured rubber to copper foil. A related material, crosslinked polyesters filled with kaolin, have attractive dielectric properties but unattractive peel strengths and other fabrication problems.
Polyphenyleneoxide (PPO, APPE, PPE) based resin systems that are cured systems of low molecular weight PPO and epoxy resins have some process limitations (U.S. Pat. Nos. 5,043,367; 5,001,010; 5,162,450) for high speed digital or high frequency applications. Their loss tangents in the gigahertz frequency range are reported to be in the 0.006-0.008 range. This is an incremental improvement over standard epoxy. Secondly, their lack of flow is a serious constraint.
Very low loss solutions include PTFE based materials and optical interconnects. Solutions containing pure PTFE based adhesive layers have the disadvantage that these materials need to be processed at temperatures exceeding 700° F. (fusion bonding, 371° C.). There are fabricators today building multilayer structures based on fluorinated resin systems. Most fabricators do not have equipment capable of pressing at these temperatures, nor are the extended heating and cooling cycles attractive to fabricators. High temperature pressing on a 34 layer count stackup could result in decreased reliability of plated through holes, PCB warping, and copper pad distortion. In high speed digital applications, via holes are a real source of signal loss. The alternative is very high layer count boards. The number one obstacle for high speed digital applications is the high layer count stack-up that encourages OEMs to source board materials that are process friendly. For high speed digital applications, the high frequency materials will be separated from the standard FR4 lower frequency layers. This leads to multiple lamination cycles. Fabricators prefer to press laminates relatively quickly at conventional epoxy pressing temperatures below 350° F. (177° C.) and have scaled their pressing capacity so that it is not a bottleneck in the entire printed circuit board fabrication process. Thus FR-4 is a material of choice. However, increasing operating frequencies demand materials having lower loss characteristics. Therefore, a composite that enables multilayer lamination at epoxy processing temperatures that has a minimum component of a hydrocarbon resin is especially desirable.
Disclosed in this invention is a fluoropolymer coated fiberglass composite that is used as the component to deliver low signal loss properties. The fluoropolymer coated fiberglass composite is then surface treated to enable it to bond to other surfaces. Surface treatment is conducted on the nanometer scale in order to maintain the desirable bulk properties of the fluoropolymer. A thin layer of a thermosetting resin which may or may not contain a ceramic filler (refer to FIG. 1) is then applied to the surfaces of the chemically modified sheet of fluoropolymer coated glass. Although the thermosetting resin represents a compromise to the otherwise good electric properties of the PTFE coated fiberglass, the thermoset enables the manufacture of a multilayer laminate at conventional epoxy processing temperatures. The thermosetting resin is partially cured (B-staged) during the application of the thermoset onto a fluoropolymer composite comprising a substrate selected from woven fabric, non-woven or a polymeric film. The electrical properties of the resulting prepreg is then determined by the ratio of the coated thermosetting resin to the fluoropolymer coated fiberglass starting material. It is preferred to limit the amount of thermosetting resin to just enough to fill the spaces between the copper traces of the inner layers and still obtain a good bond.